The present invention relates to a technology effective for application to a PLL (Phase-Locked Loop) circuit provided with a plurality of VCO (Voltage-Controlled Oscillators) and capable of performing switching between oscillation frequencies, and a technology effective for application to a PLL circuit used as a local oscillator for generating an oscillation signal having a predetermined frequency, which is merged with a receive signal and a transmit signal in a wireless communication apparatus such as a portable or cellular telephone or the like capable of transmitting and receiving signals lying in plural bands, and to a wireless communication system using the PLL circuit.
As a mobile system like a portable or cellular telephone, may be mentioned, dual-band type cellular telephones capable of handling signals lying in two frequency bands, like, for example, a GSM (Group Special Mobile) using a band which ranges from 880 MHz to 915 MHz and a DCS (Digital Cellular System) using a band which ranges from 1710 MHz to 1785 MHz. In the cellular telephone, a PLL circuit is used as a local oscillator for generating an oscillation signal having a predetermined frequency, which is merged with a receive signal and a transmit signal. However, the cellular telephone capable of handling the signals lying within the two frequency bands much different from each other as described above encounters difficulties in covering the two frequency bands by one VCO from the viewpoint of circuit's characteristics. Thus, VCO corresponding to their frequencies are provided and selected according to a used frequency band.
FIG. 5 shows an example of a configuration of a conventional PLL circuit employed in a dual-band type cellular telephone. The PLL circuit comprises a divider 11A which frequency-divides a reference frequency signal TCXO like 13 MHz into a signal R (hereinafter called a “reference side pulse”) of about 200 KHz substantially equal to a channel interval, a divider 11B which frequency-divides a feedback signal F sent from either one of VCO into a pulse N (hereinafter called a “feedback side pulse”) having a frequency of 200 KHz identical to the reference side pulse R, a phase comparator 12 which compares the phase of the feedback side pulse N with that of the reference side pulse R and detects the difference in phase therebetween, a charge pump circuit 13 which delivers an electrical charge corresponding to the detected phase difference and draws or discharges it, a loop filter 14 which generates a voltage corresponding to the electrical charge supplied from the charge pump circuit 13, two voltage-controlled oscillators (VCO) 15A and 15B each of which oscillates at a frequency corresponding to the generated voltage, and a selector switch 16 for selecting and feeding back oscillation outputs of these voltage-controlled oscillators 15A and 15B.
Incidentally, the interval between channels (frequency bands) is 200 KHz in the PLL circuit employed in the cellular telephone. In order to generate a local oscillation signal merged with a transmit/receive signal and identical to each selected channel in frequency from the PLL circuit for the purpose of selecting a desired channel from plural channels, a variable divider capable of changing a division ratio is used as the feedback side divider 11B. When the switching between the channels is performed, the division ratio of the variable divider 11B is changed to another according to a control signal sent from a system controller.
When a used band is changed from a GSM band to a DCS band or from the DCS band to the GSM band, the selection of the division ratio of the variable divider 11B according to the control signal sent from the system controller and the switching between the outputs of the voltage-controlled oscillators (VCO) 15A and 15B by the switch 16 are substantially simultaneously carried out. Since, at this time, the time required to obtain the stabilization of a VCO output owing to the output changeover of the switch 16 is longer than a response time of a division output based on the selection of the division ratio of the variable divider 11B, the switching to the VCO is normally carried out on ahead.